Saginaw 3 speed Speedo Drive Gear?
 

How many teeth and what size is the speedo drive gear ?

8, large?

I was switching from a 21 tooth "driven" gear to a 20 tooth, didn't work. I assume the 20 is to small for the drive gear. The 21 tooth works but I am a few MPH off on speedo so a 20 should be right

it had a 23 tooth driven but I put on larger tires

Re: Saginaw 3 speed Speedo Drive Gear?
 

There are two different worm gears possible on the tailshaft. The 1.84" OD gear uses 17 to 22 tooth pencil gears that are .810" OD. The1.76" OD gear uses 22 to 24 tooth pencil gears that are .870" OD. The 20 tooth blue, or 21 tooth red are both used with the 1.84 OD worm gear. The smallest that will work with the 1.76" gear is the 22 tooth green. If you have a 1.76 OD worm gear on your trans tailshaft and the 22 tooth green gear reads too slow then you have the wrong worm gear on the tailshaft. Only way to fix that is remove the trans, pull the tailshaft housing, the gear, and press on another gear. Or, i believe there are aftermarket gear conversion devices that allow you dial in a change although I've never used one of those.

This is the information for the typical 60 era speedometer gears.
If you send me your tire diameter, or better yet the revolutions per mile from the manufacture, and the differential gear ratio I will do the gear math for you.
Remember the GM calculations were typically off some. The error was typically on the fast side for liability reasons.

Re: Saginaw 3 speed Speedo Drive Gear?
 

The gear conversion devices are called ratio adapters.
Like this one.
There are different styles.
Straight, offset, 90 degree etc.


https://transmissioncenter.net/shop/...ratio-adapter/

Re: Saginaw 3 speed Speedo Drive Gear?
 

Thanks for the comment's guys...I am aware of the small & large size drive gears. Its stock 3, Saginaw, speed and 3:73 gears. The larger tires require a driven gear adjustment. I went from 22 (original) to 21 (red) driven gear but it needs a 20 tooth. Speedo is reading about 4mph slower than actual using GPS app on my phone.

I have dealt with this on other cars I have. I assumed my drive gear is the 1.84 8 tooth which is why I asked the ? on what a stock Saginaw came with with a truck with 3:73 gears

When I put the 20 tooth in the speedo would intermittently jump up some but not read nothing consistent. To me it seems like the teeth are just missing the drive gear. Its a GM driven gear.

This link (below) is what I used to determine what tooth gear I need, which said 21....although actual is a few mph off. I will measure tire diameter to confirm if it is what tire spec sheet says...in the past those are not accurate.

http://www.stl-vettes.com/65Vette/GearCalc.htm

I also have one of those right angle speedo gear reducers...it is not what I need for this

Re: Saginaw 3 speed Speedo Drive Gear?
 

What color was your original 22 tooth driven gear? If it was green, then it was the larger .870 diameter - which works with the 1.76" drive gear. There is also a 22T .810 gear which I think is gray and works with the larger 1.84" drive gear. I think 22 is the lowest .870 driven gear you can get - I'd like to find a 19T gear in .870 diameter to get my speedometer closer to being accurate but I don't think they exist..?

Re: Saginaw 3 speed Speedo Drive Gear?
 

Green is my 22 tooth gear

The 21 tooth, Red works but the 20 tooth, blue does not. Seems like the drive gear teeth just don't contact the smaller 20 tooth gear

My tires are 28.9" tall (235/75 R15 BF Goodrich) with 3:73 gears. RPM at 70 mph is 3k rpm. Speedo is about 4-5 mph off (reads slower than actual) so a 20 tooth would be ideal....although the online "calculator" says 21 tooth

Re: Saginaw 3 speed Speedo Drive Gear?
 

The 235/75R15 BFGoodrich All-Terrain T/A KO2 has a diameter of 28.9", a width of 9.3", mounts on a 15" rim and has 721 revolutions per mile.

Using the, factory provided, Rev. Per. Mile is more accurate. Because of the flat spot where the tires contact the pavement they act like slightly smaller tires.

My calculations show a 7.3% error with the 8 & 21 gears.
My calculations show a 2.4% error with the 8 & 20 gears.

I find it easier to do the % error and use that to correct the gears. However, one need both actual speed and the indicated speed at to calculate the % error.

Below are some other options using the gear ratios between the two speedometer gears:

Re: Saginaw 3 speed Speedo Drive Gear?
 

^^, good info

I have the BF Goodrich Advantage, not All terrain. It is spec at 28.9" diameter though

What I don't get is why the 20 tooth won't work with the large 1.76 drive gear. Its a GM part, not repop gear. GM # 3987920

Maybe the 20 doesn't work with a 8 tooth, if that is what the Saginaw 3 speed has, drive gear and needs the 9 tooth??

Re: Saginaw 3 speed Speedo Drive Gear?
 

I am kind of in the same boat. My original green gear was worn and not making contact any longer (not sure why the transmission guy didn't catch that and now I am worried what other issues he didn't correct in my 3 speed). I bought the brown gear (3987918) because the guy said it was the same for both applications. Lots of transmission places are listing the same. I really dont want to pull this out at this point or buy a $120 ratio corrector. Just need to find that hens tooth gear. Mine was reading about 10 mph slow at 45mph.

Re: Saginaw 3 speed Speedo Drive Gear?
 

17 PURPLE
18 BROWN
19 WHITE
20 BLUE
21 RED
22 GRAY
THESE GEARS HAVE A .780" DIAMETER HEAD AND A 1.760" STEM LENGTH.
THEY ARE FOR USE WITH DRIVE GEARS THAT HAVE AN 1.840" OD.

GM actually made two gears for the tail shaft both with an 8 tooth count but in two different outside diameters. In the early years 3 and 4 speeds these were metal gears with an interference fit on the shaft. Only way to tell for sure which one you have it to measure the OD. I know for sure my 1970 3 speed had the metal gear installed on the tail shaft. The speedometer gears were the only parts I kept from the 3 speed transmission.
Confusing isn't it?